8 Memoirs of the Indian Museum. [Voi,. I, 



THE CHARACTERISTICS OF IMMATURITY IN RATS. 



I have been bold enough to make the heading of this section apply to rats in general, 

 though I have examined the young of only 2 genera and 5 species, for the reason that the 

 distinctions to be described are common to many, if not most, of the mammalia, and 

 are very marked in man and the anthropoid apes. In the young of mammals it is 

 found that in early immaturity the cranium is globular and large in proportion to the 

 facial part of the skull, also that the feet and ears tend to be large compared to the 

 general body dimensions. In the rat, as will be shown, those characteristics are very 

 definitely displayed, and in addition the pelage of the young is very distinct from 

 that of the mature rat. The importance of clearly defining the marks of youth is 

 brought home to one when one comes across a description of a new rat founded perhaps 

 on the examination of a single specimen. Eet me quote the instance of Mus blan- 

 fordi as an example, where the type was an immature one having only 68 per cent, of 

 an adult length. Had the value of proportional measurements then been recognized, 

 Thomas would at once have seen that the specimen was immature and would not 

 afterwards have had to alter the drawing of the skull and his original description, 

 admitting that the slope of the anterior zygoma root was simply a character of im- 

 maturity, not a specific character as at first laid down. That twenty- five years later 

 the same factor of error is still given full play will be seen from a subsequent note on 

 a so-called specimen of Nesokia hengalensis from Kilakarai, G. of Manaar. I will first 

 lay down the law of proportional body measurements. 



// the length of the hind foot is as much as 30 per cent, of the total body-length, then 

 the rat is immature, is just cutting, or has just cut, its third upper molar, and has 

 {approximately) only ^o to 60 per cent, of its ultimate body-length. 



The foot of a mature rat is only about 20 per cent, of the body-length and may, 

 in a very large and old rat, fall as low as 16 per cent. The ear is a more variable char- 

 acter, so that it is more difficult to lay down a gerieral law ; but it will be found that 

 there is a difference of nearly 5 per cent, between the ear of a rat of the age indicated 

 and that of a mature one. The tail is a still more variable character, but even in it the 

 tendency is for the proportions of youth to exceed those of maturity. The following 

 is a table of typical instances from each species, including Mus blanfordi from Thomas's 

 description ;— • 



